1. Field of the Invention
The present invention relates to the field of wireless communication systems. More specifically, the invention relates to architectures and methods for high capacity WLAN access points with multiple antennas.
2. Description of the Related Art
Many mobile devices, such as cellular phones for example, are capable of operating in two modes. In one mode, the mobile device is capable of linking to a cellular network having a cell range on the order of kilometers thereby allowing the user of the mobile device to maintain communication while traveling in a car or train. In another mode, the mobile device is capable of linking to short-range wireless technologies, such as for example wireless local area networks (WLANs) or Bluetooth.
The growing use of these dual-mode mobile devices coupled with the perceived need for high data rate access to a network while in public places such as airports or coffee shops, for example, create bottlenecks at information hot-spots where a WLAN cell lacks the capacity to service all the access requests from the users within the WLAN cell.
To illustrate bottlenecks at the access point, a discussion of network communications protocols are necessary. Network communications may be modeled on a layered architecture known as a protocol stack. The layered structure allows separate development teams to work on different layers while maintaining interoperability among the various network hardware and software vendors. The Open Systems Interconnection (OSI) reference model separates network communications into seven distinct layers while the older TCP/IP protocol uses a four-layer stack. Both architectures have a top application layer that provides network services to applications requesting such services and both architectures have a bottom layer that handles the encoding of binary data, conversion of the binary data into physically measurable quantities such as voltages or light pulses, and transmission of the measurable quantities over a transmission medium such as air or cable.
Wireless local area networks (LANs) have at least a portion of the network that does not use cable as the transmission medium. The wireless LAN protocols are defined in OSI Layer 1 and OSI Layer 2. OSI Layer 1, also referred to hereinafter as the physical layer, defines the protocols for the encoding of binary data, conversion of the binary data into physically measurable quantities such as voltages or light pulses, and transmission of the measurable quantities through the air.
OSI Layer 2, also referred to hereinafter as the data link layer, defines the protocols for providing basic packet addressing services, checking transmitted packets for errors, and arbitrating access to the network.
Several standards exist for wireless LAN technologies such as IEEE 802.11, HiperLAN/2, and Bluetooth. For purposes of illustration, the mobile devices are assumed IEEE 802.11a compliant, however, it should be understood that the scope of the present invention is not limited to IEEE 802.11a compliant devices and may encompass devices based on other wireless LAN standards.
The IEEE 802.11a standard defines operations using orthogonal frequency division multiplexing (OFDM) modulation in the 5 GHz band. The standard supports a maximum data transfer rate of 54 Mbps. The maximum data transfer rate of 54 Mbps may become a bottleneck when many devices attempt to communicate with the same access point in a crowded café, for example.
FIG. 1 is a diagram of the physical and data link layers of the network protocol stack that indicates the scope of the IEEE 802.11a standard. As indicated in FIG. 1, the IEEE 802.11a standard, indicated by double-ended arrow 100, includes the physical layer 140 of OSI Layer 1 180 and part of the data link layer of OSI Layer 2 185. The data link layer 185 is divided into two sublayers; the medium access control (MAC) layer 145 and the logical link control (LLC) layer 147.
The LLC layer 147 is defined by a standard such as, for example, IEEE 802.2 standard and provides addressing and data link control that is independent of the topology, transmission medium, and medium access control protocols of the underlying layers.
The MAC layer 145 provides access control functions for the shared transmission medium. Access to the wireless medium in the IEEE 802.11a and HiperLAN2 standards are based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The MAC layer 145 can operate in two modes: Distributed Coordination Function (DCF) and Point Coordination Function (PCF).
FIG. 2 is a timing diagram illustrating the communication exchange between two wireless devices using DCF. Under CSMA/CA, the mobile device (MD) or access point (AP) listens to a channel to determine if the channel is idle. If the channel is carrying a transmission, both the MD and AP continue listening to the channel. Once the channel becomes idle, the MD and AP waits a predetermined time referred to as Distributed coordination function Inter Frame Space (DIFS) 210. In order to avoid collisions with other devices, the transmitting device waits an additional backoff period 220. The backoff period is a randomly generated integer representing the number of timeslots the device will wait before attempting to transmit a packet on the channel. Backoff timer decrements the randomly generated integer until the channels becomes busy again or the time reaches zero. If the channel becomes busy during the backoff period, the device monitors the channel until it senses the channel is idle. Once the channel is idle, the device waits for one DIFS period before resuming the countdown on the backoff timer. When the backoff timer reaches zero and the channel is still idle, the device begins packet transmission 230. The receiving device receives the transmitted packet and checks the packet to determine if there were any transmission errors. If there are no transmission errors, the receiving device waits for one Short Inter Frame Space (SIFS) 235 before transmitting an acknowledgement (ACK) 240. The SIFS period is less than the DIFS period and prevents other devices from transmitting on the channel before the ACK is transmitted.
In PCF mode, an AP coordinates the resource management of the channel. The AP sequentially polls each of the devices in its cell by sending a polling message to each device. If the AP has data for the device, the data may be including in the polling message to the device. In response to the polling message, the polled device either sends an ACK to the AP or transmits data to the AP if the device has data for the AP. The response to the polling message is transmitted within one SIFS interval. If the AP does not receive an ACK or data from the polled device within one SIFS interval, the AP sends a polling message to the next device within one Priority Inter Frame Space (PIFS) interval, which is longer than a SIFS interval but shorter than a DIFS interval.
Although DCF and PCF may be used within the same cell, once the AP captures the channel in PCF mode, the AP will control the channel until the AP releases the channel because the SIFS and PIFS intervals are shorter than the DIFS interval.
The number of APs are minimized to provide sufficient coverage of a particular area. But, as the deployment of mobile devices increase, a bottleneck will develop from the system capacity and performance in the AP. More particularly, in current wireless LAN systems, the maximum data rate of 54 Mbps is shared among K mobile devices in the coverage area. The efficiency of media access using the CSMA/CA protocol drops significantly when K is large.
Therefore, there remains a need for devices and methods that increase the capacity and performance of the access infrastructure when deployed in public areas. Furthermore, such devices and methods should remain compatible with existing standards such as, for example, the IEEE 802 family of standards or the HiperLAN/2 standards.